The present invention relates to systems for driving piezoelectric actuators and piezoelectric motors and more particularly to the electronic circuitry of such drivers.
Piezoelectric materials are characterized in that when they are subjected to electrical fields they can be made to deflect; i.e., produce mechanical motion. Also when mechanical stress is applied to piezoelectric elements so hat they undergo displacement they generate electrical signals. These characteristics are the reason that piezoelectric materials are useful in applications ranging from sensors to mechanical motors. Examples of piezoelectric elements being used as motors is shown for example in U.S. Pat. Nos. 5,714,833 and 5,777,423; which patents are assigned to the assignee of the invention of this application. Piezoelectric motors are characterized by their mechanical simplicity. They have very few parts, no separate moving parts, and there are no critical mechanical components such as gears, shafts, bearings etc. Consequently, piezoelectric motors are relatively inexpensive and highly reliable.
A piezoelectric element represents an electrical reactive load (mainly capacative) that requires an AC signal of substantial voltage amplitude to cause the mechanical displacement. The required voltage amplitude is generally in the range of a few hundred volts (RMS). For effectively operating the piezoelectric element as a motor or actuator a drive circuit applies voltage thereto of a specific frequency with low harmonic distortion. For the best results, i.e. maximum displacement, the specific frequency should equal the mechanical resonant frequency of the piezoelectric element. A simplified circuit that could be used to drive a piezoelectric motor is shown in FIG. 1A, while such a motor is illustrated in FIG. 1B. This method required two sources and two matching (high-Q) resonant circuits (not shown).
A preferred manner of actuating bidirectional operation of the piezoelectric element provides one side of the element with a pair of electrodes that are connected to a voltage source, such as a switch mode inverter. An opposite side of the piezoelectric element has a single common electrode. The common electrode is grounded or connected to ground by passive or active elements so that current from each of the pair of electrodes flows in opposite directions to cause bidirectional displacement of the piezoelectric element.
The drive circuits for piezoelectric motors basically provide AC voltages to the electrodes according to the directional movement required or desired. Typical prior art drive circuits for actuating the piezoelectric elements to provide movement in a selected direction or selected directions comprise separate inverter circuits coupled to the piezoelectric element through resonant circuitry attached to each of the elements high side electrodes. The prior art utilizes such multiple inverters to cause directional motion of the piezoelectric motors without any separate switching units. Other prior art devices utilize a single high voltage source and switching circuitry to switch the source between different electrodes. This methodology requires high voltage capable switching circuitry.
A major disadvantage of the prior art drivers is the complexity of the circuitry that is used. For example, a commonly used driver comprises a bridge type inverter circuit (see FIG. 2) that requires six power switches, and two high Q resonant circuits. Other prior art drivers require only one high Q resonant circuit The reduction in the number of high Q resonant circuits is accomplished by placing the high Q resonant circuit in series with the common electrode. However, six power switches are still used in that prior art device (see EP 0712 170 A1), which application is assigned to the assignee of this application. In FIG. 7 of that device, four switches are used for displacement directional control with two switches for each of the selectable directions. The high Q resonant circuits are sensitive to the accuracy of the circuit component and to the frequency of the applied voltage. As a practical solution for overcoming this sensitively, in the prior art the Q of the drivers was often lowered and the input voltage from the prior art drivers was raised Extra DC/DC converters were required to enable operation with lower input voltages. Consequently, in the prior art, while the piezoelectric motors are simple the driver circuitry has until now been complex, especially when driving piezoelectric motors in two directions.
Accordingly, it is an object of some preferred embodiments of the present invention to provide drivers for piezoelectric elements that are reliable, simple and of low cost.
It is an object of some preferred embodiments of the present invention is to provide drivers for piezoelectric elements that operate the piezoelectric elements as actuators or motors bi-directionally with a minimum of components.
It is an object of some preferred embodiments of the invention to provide bi-directional drivers for piezoelectric motors or actuators in which a single high voltage unswitched drive circuit is used for both direction of motion.
It is an object of some preferred embodiments of the present invention to provide bi-directional drivers for piezoelectric motors or actuators in which the direction of motion is changed by switching circuitry located at a low voltage connection of the control circuitry.
It is an object of some preferred embodiments of the present invention to provide drivers for piezoelectric elements that utilize soft switching so as to minimize switching losses and increase the overall efficiency of the piezoelectric driver unit and to improve electromagnetic compatability.
It is an object of some preferred embodiments of the present invention to provide a piezoelectric driver operated from a low DC voltage source without additional complexity.
It is an object of some preferred embodiments of the present invention to provide a discrete bidirectional switching circuit used in conjunction with an AC voltage provided for operating a piezoelectric motor or other AC load.
Some prior art driver circuits for driving piezoelectric motor firm a DC voltage source, integrally, combined switching and inverter circuitry. That is the inverters did the switching of the inverter output to the piezoelectric element""s electrodes without any discrete switching circuitry. The present invention separates the inverter and switching portions of the driver circuitry by providing discrete switching circuitry. Surprisingly, the result is a circuit for driving the piezoelectric element bidirectionally of decreased complexity using fewer components and operating more reliably.
In accordance with one aspect of the invention, a driver circuit for driving a piezoelectric type motor comprises an inverter circuit for providing an oscillating voltage to cause a mechanical displacement of a piezoelectric element and a separate switching arrangement for selecting the direction of said displacement. The driver circuit in the invention of this application delivers the driving voltages to the piezoelectric element at the mechanical resonant frequency of said piezoelectric element.
In a preferred embodiment of the invention a discrete switch arrangement includes high frequency switches for selectively applying voltages across said piezoelectric element to cause mechanical displacement of the piezoelectric element in a selected direction or selected directions. In preferred embodiments of the invention, the switches are on a low voltage side of the connection to the piezoelectric element.
There is thus provided, in accordance with a preferred embodiment of the invention, a micromotor comprising:
a piezoelectric element including a common electrode and a plurality of other electrodes formed thereon and including at least a first and a second electrode group, each group including at least one electrode, where the piezoelectric element causes motion in a first direction when a voltage is applied between the first electrode group and the common electrode and wherein the piezoelectric element causes motion in a second direction when a voltage is applied between the second electrode group and the common electrode;
an voltage source that electrifies the common electrode; and
at least two switches separately connected between the first and second electrode groups and a low voltage, said switches being activatable to connect one of said first and second electrode groups to the low voltage to cause selective motion in the first or second directions.
Preferably, the low voltage is substantially ground. Preferably, the applied voltage is an AC voltage.
In a preferred embodiment of the invention, where the piezoelectric element comprises a rectangular piezoelectric plate having fist and second faces, the common electrode is formed on the first face and the first and second groups of electrodes are formed on the second face. Preferably, the first and second groups of electrodes each comprises two electrodes situated in opposite quadrants of the second face.
Preferably, the micromotor comprises a motive surface and motion is induced in a surface of a workpiece pressed against the motive surface when the piezoelectric element is electrified as aforesaid.
There is further provided, in accordance with a preferred embodiment of the invention, a micromotor comprising:
an ultrasonically vibrating element; and
a drive circuit comprising:
an oscillating voltage source connected to and electrifying at least one electrode of said ultrasonically vibrating element to cause a mechanical displacement of a portion thereof; and
a discrete switch arrangement attached to at least one additional electrode of said ultrasonically vibrating element to which said oscillating voltage is not connected which switch arrangement selects the direction of said displacement.
Preferably, the ultrasonically vibrating element comprises a piezoelectric element
Preferably, the at least one additional electrode comprises a plurality of electrodes applied to a first face of said vibrating element; and the at least one electrode comprises a common electrode applied to a second face of said element. Preferably, the discrete switch arrangement selectively applies voltage between a first group of said plurality of electrodes and said common electrode to cause displacement in a first direction, said first group including at least one electrode. Preferably, the discrete switch arrangement selectively applies voltage between a second group of said plural of electrodes and said common electrode to cause displacement in a second direction, said second group comprising at least one electrode.
In a preferred embodiment of the invention, the discrete switching arrangement comprises a pair of switches connected to apply voltages across said element to drive current through said element, and controls for selectively operating said switches to select the direction of said displacement. Preferably, the switches of said discrete switching arrangement comprise solid state switches, preferably, transistorized switches and most preferably, Mosfet transistors.
In a preferred embodiment of the invention, the discrete switch arrangement comprises:
a first Mosfet connected between a first voltage and said first group of electrodes;
a second Mosfet connected between said first voltage and said second group of electrodes, said common electrode being connected to a second voltage, and
a control that selectively operates said Mosfet switches to selectively apply said first voltage to the first electrode group or to said second electrode group.
Preferably, the micromotor includes a source of control voltages selectively applied to the gates of said first and second Mosfet transistors for selectively switching said first or said second Mosfet transistors from the non conducting state to the conducting state.
Preferably, the micromotor includes a pair of diodes, one of which is connected across each said Mosfet transistor. Preferably, the diodes are connected such that they conduct DC current toward the micromotor.
In a preferred embodiment of the invention, the transistor is off, one end of the Mosfet is at a DC voltage equal to the peak of the oscillating voltage and the oscillating voltage is impressed across the Mosfet transistor, such that the voltage across transistor is substantially unipolar.
In a preferred embodiment of the invention, the source comprises an inverter, preferably, a forward-flyback inverter.
Preferably, the forward-flyback inverter comprises:
a magnetic element having a primary winding and a secondary winding, said primary winding being connected between a first inverter voltage and one side of an inverter operating switch, the other side of said inverter operating switch connected to a second inverter voltage so that when said inverter operating switch is in the conductive stage, current flows through said primary and when said inverter operates switch is in a non conductive state substantially no current flows through said primary; and
a control voltage source which selectively turns said inverter operating switches switch on or off, the secondary of said magnetic element being connected to said discrete switching arrangement.
Preferably, the inverter operating switches comprise solid state switches, preferably transistorized switches most preferably, Mosfet transistor, and further comprises circuitry that causes said inverter output to resonate at substantially the mechanical resonant frequency of said piezoelectric element.
In a preferred embodiment of the invention, the circuitry that causes said inverter output to resonate at substantially the mechanical resonant frequency of the piezoelectric comprises a capacitor bridging said switch and in series with the pi of the magnetic element, said capacitor operating in conjunction with the capacitance of said ultrasonically vibrating motor to turn said magnetic element to resonate at substantially the mechanical resonant frequency of the motor.
In a preferred embodiment of the invention, the inverter is a push-pull inverter.
Preferably, the push-pull inverter comprises a transformer, having a primary and a secondary;
a series inductor connecting a first inverter input to a mid part of the primary of said transformer,
the secondary of said transformer connected to said discrete switching arrangement and one side of the primary of said transformer being connected through a first push-pull inverter switch to a second inverter input,
the other side of said primary of said transformer being connected through a second push-pull inverter switch to said second input.
Preferably, the fist and second push-pull switches comprise solid states switches, more preferably, transistorized switches and most preferably, Mosfet type switches.
In a prefer embodiment of the invention, the capacitance of said ultrasonic motor and the inductances of the series inductance and of said transformer match the electrical circuit to the mechanical resonance of said piezoelectric element. Preferably, the first or the second push-pull inverter switches are each selectively operated by control voltages, preferably, square wave voltages.
In a preferred embodiment of the invention, the push-pull inverter includes a buck section for controlling the amplitude of the voltage connected to said primary of said transformer. Preferably, the buck section includes:
a series buck section switch connected between the first input of the inverter and the series inductor;
a diode connected between an output of the buck section switch and said second input of the inverter and,
a control voltage operative to cause the buck section switch to conduct.
In a preferred embodiment of the invention, the second input is ground. Preferably, the first input is electrified with a DC voltage.
There is further provided, in accordance with a preferred embodiment of the invention, a method of supplying switchable AC power to a load comprising:
connecting a fist terminal of an AC power source to one side of the load;
connected a Mosfet transistor between a second terminal of the AC power source and the other side of the load; and
selectively supplying power to the load by applying a voltage between a gate of the Mosfet and the second AC terminal.
Preferably a diode is connected across the Mosfet. Preferably, the diode is connected such that it conducts current between the second terminal and the other side of the load.
In a preferred embodiment of the invention a capacitor is placed in series with the load, which preferably, does not include a DC blocking capacitor.
In accordance with a preferred embodiment of the invention, when the transistor is off, one end of the Mosfet is at a DC voltage equal to the peak voltage of the AC source and AC voltage of the AC source is impressed across the Mosfet transistor, such that the voltage across transistor is substantially unipolar.